In the field of art conservation, one of the guiding principles is that of reversibility. The American Institute for Conservation of Historic and Artistic Works (AIC) is the only national membership organization in the United States dedicated to the preservation of cultural material. Conservation treatment guidelines pertaining to ‘Compensation for Loss’ state that “any intervention to compensate for loss should be documented in treatment records and reports and should be detectable by common examination methods. Such compensation should be reversible and should not falsely modify the known aesthetic, conceptual, and physical characteristics of the cultural property, especially by removing or obscuring original material.” See, http://www.nps.gov/training/tel/Guides/HPS1022_AIC_Code_of_Ethics.pdf. At minimum, “compensation must be reversible, using chemical and/or mechanical methods that will not adversely affect the remaining original material, unless this jeopardizes structural stability.” In preservation, this idea of reversibility is important to both preserve the original intent of the artist, as well as to periodically assess and maintain structural stability while preserving the visual unity of a work of art. Coatings on art should ideally provide a physical, transparent, removable barrier between the outside world and the artwork beneath. In this sense they are intentionally sacrificial and their intended usages range from protecting artworks from natural processes (e.g., ultraviolet irradiation, atmospheric gases) to vandalism (e.g., graffiti, tagging).
In application, restoration and protective coatings for fine art surfaces must have excellent aging properties and remain soluble in appropriate solvent systems so that they may be replaced or repaired periodically. Several examples of these kinds of solvent reversible coatings that have been adopted for, or function well in museum, gallery, and indoor spaces can be cited (http://nautarch.tamu.edu/CRL/conservationmanual/File2.htm); these include poly(vinyl acetate), polyvinyl alcohol, reduced ketone resins, reduced aldehyde resins, and poly acrylates to name just a few. Typical solvents for the application and removal of artistic clear coatings derived from these type of polymers (e.g., ‘synthetic varnishes’) include (but are not limited to) low boiling point aliphatic hydrocarbon solvents, toluene, xylene, alcohols, mineral spirits (e.g., turpentine), acetone and mixtures thereof. Solvent systems are generally highly volatile, relatively toxic, and their use and proper disposal is subject to federal, state, and local standards and regulations (e.g., OSHA, NIOSH). Conservation procedures often involve the application and removal of materials that may endanger the health and safety of conservation professionals, other contracted persons involved in carrying out procedures, and the public in outdoor settings. With increased environmental awareness, there is a strong desire to develop improved technologies that would allow use of water-based coating systems to protect art and artifacts in general.
Art that is displayed or is created specifically in outdoor contexts (mural arts for instance) also require coatings to be applied to them for the same reasons cited above. Coatings applied on these materials must also meet the requirement of reversibility, as well as the aesthetic, and protective functional demands but in extreme environments very different from the museum or gallery context (cycling RH and temperatures, freeze/thaw cycling, wind, rain and other atmospheric conditions), and must comply with stringent environmental requirements.
Many outdoor murals are constructed by artists using acrylic colors. For example, see http://www.goldenpaints.com/technicaldata/murals.php. Unfortunately, many acrylic colors have been found to weather poorly and degrade rapidly in outdoor contexts. See, http://www.getty.edu/conservation/publications_resources/newsletters/18_2/feature.html. It should be noted that artist's acrylic paints have been found to be easily and substantially extracted with a wide array of solvents and with pure water, and water with only limited alterations made to it. (Smith, G. D. 2007, Aging characteristics of a contemporary acrylic emulsion used in artists' paints, In Modern Paints Uncovered, eds. T. Learner, P. Smithen, J. Krueger, and M. Schilling, 236-246, Los Angeles: Getty Conservation Institute; Zumbühl, S., F. Attanasio, N. Scherrer, W. Müller, N. Fenners, and W. Caseri, 2007, Solvent action on dispersion paint systems and the influence of morphology—changes and destruction of the latex microstructure, In Modern Paints Uncovered, eds. T. Learner, P. Smithen, J. Krueger, and M. Schilling, 257-268, Los Angeles: Getty Conservation Institute).
Recent research has identified that aqueous systems whereby such solution parameters as pH, overall ionic concentrations, and specific ion effects, as well as adjuvant materials like surfactants, chelators, buffers, etc., can mitigate the swelling, extraction, and general degradation of these kinds of paints associated with water alone and are therefore useful properties to manage in aqueous solutions to aid both in the application as well as in the removal of potential protective coatings. (R. Wolbers, A. Norbutus and A. Lagalante “Cleaning of Acrylic Emulsion Paints: Preliminary Extractive Studies with Two Commercial Paint Systems”, New Insights into the Cleaning of Paintings, Proceedings from the Cleaning 2010 International Conference, Universidad Politécnica de Valencia and Museum Conservation Institute, Eds. Marion F. Mecklenburg, A. Elena Charola, and Robert J. Koestler, Smithsonian Contributions to Museum Conservation, no. 3 (2013); C. E. Dillon, A. F. Lagalante and R. C. Wolbers “Aqueous cleaning of acrylic emulsion paint films. The effect of solution pH, conductivity and ionic strength on film swelling and surfactant removal” Studies in Conservation 57(1), 52-62 (2014)).
While no coatings have been developed specifically for outdoor fine art applications over acrylic paints, there are coatings that are nonetheless being used to that effect. Solvent based, and solvent (only) removable coatings are at present commercially available and being used by artists to coat their own work. Golden Acrylic Colors ‘GEL Soft Gloss, MSA Varnish with Ultraviolet light stabilizers (UVLS) (http://www.goldenpaints.com/technicaldata/murals.php) is an example of a coating that is recommended by the manufacturer to be applied by the artists to their own work, and ostensibly removed in this manner. A 2010 mural arts practices survey (http://www.muralroutes.com/resources/Mural %20Arts %20Practices %20 Survey_final.pdf) documented the range of solvent borne coatings being applied by artists to their own works. In addition to the Gel Soft Glass MSA varnish from Golden, these have also included Varathane Diamond Polyurethane; Nova. Color Acrylic Mat Varnish, GAC-500 acrylic polymer, Benjamin Moore Stays Clear (Polyurethane), Behr Premium Plus Ultra, Deep Base Clear’, One Shot Clear Coat UV, Ronan Vinyl Cote UV Absorber Gloss, Stevenson's varnish, Aquarius Coatings Armaglaze 6000, Liquitex, Graffitex, Semigloss, Rohm and Haas' Paraloid B72, PPG's Deltron DC3000 (with DCU2060), Aquacoat's Auto Top Coat, and Adicolor's DFV clear coat. But all of these materials are essentially irreversible or only reversible in solvents that would affect the underlying paints, and must be considered irrevocably part of the art works. Several of these coatings have been tested for reversibility and have not met with any success without damage to the paints beneath. (http://www.getty.edu/conservation/our_projects/science/outdoor/index.html).
In a related art, the use of water-based, oligorneric organosiloxanes with fluorinated alkyl groups as anti-graffiti coatings is well known (US20130040058 A1). However, a problematic element which has emerged in the use of these aqueous formulations is their film-forming properties. When the known water-based anti-graffiti formulations are applied, coherent films are not always formed. Additionally, these materials, while water applicable, become intractable to water on drying/curing and can only also be reversed with solvents that would damage the paints beneath.
Said coatings however would have to be not only compatible with artist's acrylic paints, but as well with the building substrate materials they have been applied to. Building substrates also contribute to the problem of finding coating materials appropriate for mural paintings. These might include, but not be limited to: brick, mortar, cementitious, metal, wood, and ceramic materials. The variety of substrate materials, variation in their condition, variations in surface preparations on each of these materials can also contribute to applied paint and coating deterioration or failure in outdoor contexts. One of the special requirements of coatings applied to building envelopes is that they have a high moisture vapor transmission value or rate (MVTR). Without the ability to allow for moisture vapor to pass through all the materials that encase buildings (including decorative and coating materials), precipitation of soluble salts on drying, or freezing and expansion of condensed or trapped water at below 0° C. temperatures would inevitably occur. To date, it has been difficult to formulate water-based coating systems that show acceptable adhesion to underlying painted surfaces, are resistant to moisture contact yet have sufficient water vapor permeability so as not to trap water vapor that can damage the underlying artwork or building exterior in freeze/thaw cycles. The coating must withstand outdoor environments which are subject to wide variations in temperature, humidity, and solar irradiation. When exposed to nature, the coating must withstand wind, sun, hail, rain, particulates, and extremes in temperature without blistering, peeling, or cracking that would compromise the visual appearance or protection of the underlying artwork.
Aqueous coatings that could be applied and reversed with aqueous methods are more likely to meet ever increasingly stringent VOC (volatile organic compound) regulations. Local, state, and federal VOC laws restrict severely the types and amounts of solvents that are inherent in, and that can be used to both apply and reverse or re-solubilize restoration coatings (http://www.issa.com//data/File/regulatory/VOC%20Limits %20Summary %2010-25-13.pdf). And new regional VOC regulatory standards (e.g. Northeast Ozone Transport Commission (NOTC), California Air Resource Board (CARB), or the South Coast Air Quality Management District (SCAQMD)) differ from U.S. Environmental Protection Agency (USEPA) national VOC standards. In general, the new regional regulations are more restrictive in lowering the maximum amount of VOC's allowed in these areas. For instance, for the Northeast and Mid-Atlantic region the NOTC Phase I Model Rule, as of Jan. 1, 2005 for ‘industrial maintenance’ coatings is set at or limited to 340 g/L. See, (http.//www.vexcon.com/pdfs/pi100vexconvocguide.pdf). With ever increasing emphasis placed on lower VOC emissions from film forming coatings or paints, coatings having much lower VOC emissions levels are required.
Potential protective coatings that were both applied from aqueous solutions or dispersions and removable with aqueous materials would be highly desirable and better meet environmental, as well as the AIC standard of practice that requires reversibility of applied coatings.